1. Field of the Invention
The invention relates to an optical display device which is suitable in particular for use in a head-up display or a head-mounted display, such as are known for example from entertainment electronics, in order to increase the exit pupil of an image-generating system by means of diffractive elements.
2. Description of the Prior Art
Currently, in the field of man-machine interfaces, principles and devices are increasingly being developed by which computer-enhanced information of the real experiential world can be transmitted to humans. So-called augmented or virtual reality displays (VRD) are an example of this. Such displays, which may for example be configured similarly to spectacles as a head-mounted display (HMD), superimpose the image of an image-generating system, for example a micro-display, with the observer's view of the real world.
Such a head-mounted display may, for example, be produced by directing the light of an image-generating system onto a planar transparent light guide and coupling it into the light guide through a diffraction grating, the so-called input grating. The light then travels along the light guide to a so-called output grating, by which the light is coupled out again in the direction of the eye. Since the light guide is transparent, the light coupled out is superimposed with the light from the real world, which strikes the opposite side of the light guide from the eye, and is therefore focused together with it onto the retina in the observer's eye. By means of the gratings, which are also referred to as holographic optical elements (HOEs), the exit pupil of the micro-display is also enlarged and adapted to the field of view of the eye. Furthermore, planar light guides and the gratings may be configured binocularly or monocularly, depending on whether or not the input grating is provided in two parts and a second output grating is provided.
Such plane-plate HMDs are known for example from U.S. Pat. No. 6,805,490B2, which shows how such a plane-plate HMD can be produced from a plurality of layers or a plurality of subplates, so that light with a different wavelength can be used.
Since the light of the image-generating system strikes one of the flat sides of the light guide perpendicularly, in order to be coupled into the planar light guide of the HMD it must be deviated so strongly at the gratings that the angle of the light with respect to the interfaces of the planar light guide is greater than the angle of total reflection. The same applies correspondingly in the converse case of coupling out.
In order to guide as much light as possible into a particular diffraction order, so-called blaze gratings are normally used which have a sawtooth profile and, by suitable parameter selection, concentrate a majority of the light in one diffraction order by the combination of refraction and diffraction effects. A characteristic quantity of such a blaze grating is the diffraction efficiency, i.e. the fraction of light which is guided into the selected order. In order to ensure an acceptable luminous efficiency of HMDs which are usually configured as mobile devices, however, the gratings used for coupling in and out must achieve a very high diffraction efficiency.
For manufacturing reasons, the gratings used in HMDs are usually surface gratings, the diffraction efficiency of which depends essentially on the profile shape of the grating. Owing to shadowing effects, which occur with the small grating widths that are required in order to achieve the strong deviation, conventional blaze gratings are unsuitable for use in plane-plate HMDs. Binary gratings, which have an input and output efficiency of at best 30 percent, are likewise not an optimal solution. So-called oblique gratings have therefore hitherto been used, although these are relatively expensive to produce.